In the drilling of an oil, gas or gas condensate wells, drilling fluid referred to in the industry as “mud”, is pumped into the drill pipe where it proceeds out through the drill bit and up the annular space between the drill pipe and the walls of the hole and further up the annular space between the drill pipe and the casing generally used, after which it is examined at the surface for certain parameters, processed and returned to circulation. The purpose of the circulating mud is to clean, cool and lubricate the bit, flush to the surface the cuttings from the bore hole and to protect the walls of the hole until casing is inserted. The density and viscosity of the mud is carefully controlled at the surface so as to contain various pressures encountered in the hole.
As the well is drilled, gases saturated in highly pressurized fluids may be released therefrom or from a porous rock and find their way into the circulating mud forming an annular gas bubble or a gaseous pack, also called a gas kick. This gas kick may ascend to the surface, result in a modification of the buoyancy of the drilling string and can cause extensive damage if it goes undetected. The gas or liquid contained in the gas kick reduces the hydrostatic head in the annulus. If the volume of the gas kick is not excessive and if it can be detected, gas kick removal procedures may be instituted so that drilling operations may proceed with minimal disruption.
It is known in the art of drilling of oil wells that gas kicks may contain pure natural gas or may alternately contain a certain percentage of water and/or oil. All of these occurrences are referred to for the purposes of this description as a “gas kick”. Rising gas kick replaces drilling mud as it ascends to the surface of the well. This in turn leads to a decrease in a well bottomhole pressure which leads to a further increase of speed of gas kick ascendance. If not detected early, this may lead to catastrophic consequences. Sometimes a gas kick may even cause an uncontrolled blowout, which has been known to cause loss of human lives, extensive equipment damage, fires, environmental catastrophe and possible release of noxious gases.
Using acoustics for detection of the gas kick presence is known in the art. U.S. Pat. No. 4,273,212 for example discloses sending an acoustic pulse down the pipe and receive its reflection in the annular portion of the well head. Using high frequency positive acoustic pulses however does not allow full characterization of the gas kick as it only allows detection of its upper end and not allows detection of its lower end which is needed to detect its total volume. The size of a gas kick directly relates to the degree of damage that it can do when such gas kick reaches the surface. In addition, frequent pulses may not reach deep enough into the well as the dense drilling mud causes them to attenuate at fairly shallow depths. Using positive pressure pulses may also increase the risk of well rupture and therefore should be avoided if possible. Finally, continuous generation of the probing signals makes it difficult to separate them from the reflected signals.
Accordingly, reliable and safe methods and devices for detecting and fully characterizing the initial gas kick and its subsequent removal are desired.